This invention relates to chemical tracing, more particularly to the incorporation of detectable fluorine compounds into materials or a fluid medium such that the materials can be identified and distinguished from materials from other sources or such that the flow pattern of the fluid medium can be defined.
A considerable technology has developed in recent years for the identification of materials or for studying the flow patterns of fluid media. For example, U.S. Pat. No. 3,964,294 to Shair et al describes the microencapsulation of analyzable compounds not normally present in petroleum crude oil and refined liquid products thereof, the incorporation of the microcapsules in the oil or liquid products, the sampling of the oil or liquid products containing the microcapsules at some location removed from the point of origin, and the analysis of the encapsulated substance by electron capture gas chromatography. In this manner the origin of the oil containing the microcapsules can be determined based upon a preestablished code or identification of the analyzable substance with a particular source of the oil or refined liquid product. The chemical tracers in such applications are termed "taggants" since they are used to label, code or "tag" the material to be identified. In this specification, "tracing" is used as a general term; taggants are one species of tracers.
The advantage of the encapsulation is that it overcomes problems associated with injecting chemical tracers directly into a material or fluid, whose movement or flow pattern is to be traced. These problems include incompatibility of some tracers with the material or fluid to be traced, interference by or confusion with other substances in the material or fluid (such as substances having a similar chemical composition), and possible contamination of the tracers with particulate matter such as spores, pollens or other microbial substances or precipitants which interfere with or complicate the separation and characterization of the tracers.
Nevertheless, even assuming the effectiveness of the encapsulation for solving the foregoing problems, the tracer material itself must have certain characteristics for efficacy. These include unique chemical composition so that its detection profile (for example, chromatographic pattern) is associated only with the tracer, a high degree of detectability (of the order of about 1 part in 10.sup.6 to 10.sup.12 parts by volume) by available detection systems, and sufficient inertness so that the tracer is not lost due to chemical, physical or biological interactions which may be encountered in use.
Chemical inertness is especially critical when a catalytic reactor is used preliminary to electron capture detection to convert substances in the sample, other than the tracer, to products which will not interfere with the analysis. For example, it is common practice to employ a palladium catalyst reactor "clean up" system prior to electron capture gas chromatography to remove certain halogenated hydrocarbon contaminates present in the atmosphere by converting the contaminates to elemental hydrides or other non-interfering products.
Applications and embodiments of material or flow pattern tracers are described in the literature, including detection techniques. In addition to U.S. Pat. No. 3,964,294 mentioned above, the following publications are representative of the literature on this subject: P. G. Simmonds et al., "Continuous and Ultra Sensitive Apparatus for the Measurement of Air-borne Tracer Substances", Journal of Chromatography, 126 (1976), pages 3-9, describing an electron capture detector apparatus, and incorporating a reference to gas chromatography ("GC") coupled with an electron capture detector ("ECD" or "EC detector"); U.S. Pat. No. 4,304,752 describing an ECD apparatus for detection of a tracer material in the atmosphere; U.S. Pat. No. 4,256,038 describing a palladium reactor in combination with ECD and GC for the detection of perfluorocarbon taggants in blasting cap detonators; U.S. Pat. No. 4,141,692 describing ECD/GC detection of taggants in fuel compositions; and U.S. Pat. No. 3,585,845 describing ECD for detection of leaks of a gaseous substance from a vessel or pipeline, based upon the presence of a tracer gas in the gaseous material. Representative U.S. patents describing the use of tracers in other applications but not necessarily involving ECD are the following: U.S. Pat. No. 4,303,411 describing the use of fluorocarbons in oil field tracing, using .sup.19 FNMR spectroscopy; U.S. Pat. No. 4,299,709 describing tracer fluids for enhanced oil recovery, utilizing standard chemical analytical procedures; and U.S. Pat. No. 2,445,494 describing a method of determining the fluid content (gas, oil and water) of a formation or a core sample therefrom, wherein the contamination of the samples by drilling fluids is traced by incorporation of a tracer material into the drilling fluids.
As evident from the foregoing literature, fluorine compounds, and in particular perfluoro compounds, generally have the inertness and structural uniqueness required for use as tracers. Nevertheless, many perfluoro compounds decompose when it is necessary to catalytically treat a sample containing the perfluoro compound tracer for the purpose of eliminating contaminants prior to analysis to confirm the presence of the tracer material.